In many manufacturing processes it is necessary to join one or more parts made of disparate materials. Brazing is one favored technique for this task. However, because parts made of different materials will usually have different coefficients of thermal expansion (CTE), the two parts will expand by different amounts when subjected to the heat of the brazing operation. Additionally, as the parts cool from the peak brazing temperatures, they will contract by different amounts. This creates strains which can result in damage along the brazed interface between the two, now joined, parts. The higher the brazing temperature, or the greater the difference between the CTE's of the two materials, the more severe these strains become.
This problem may be particularly acute during the manufacture of high temperature heat exchangers. For example, critical components in nuclear fusion reactors are heat shields composed of carbon-carbon composite ("C--C") surfaces joined to water-cooled copper-based substrates. Because of the very high braze temperatures needed to join these materials, the order-of-magnitude difference between the CTE's of the materials causes large strains to form at the joint interface during the cooling process. The strains are frequently large enough to cause failure of the C--C part near the joint. Joints which survive the cooling process contain significant residual strains at room temperature which lead to premature failure of the joint in a thermal fatigue environment.